1. Field of the Invention
The present invention relates to fluid flow constraining devices that are used to direct or control the flow of liquids and gases. More specifically, the present invention relates to a method and apparatus for increasing the efficiency of fluid flow through a valve or other fluid flow constraining structure of the type in which the fluid flow direction is changed between the inlet and the outlet of the structure. In a specific application, the present invention is directed to right angle body valves flowing vapors or liquids in what is known as a subsonic flow condition.
2. Setting of the Invention
A right-angle body valve flowing vapors in a "subsonic" flow condition exhibits pressure at the plane of its discharge nozzle that is greater than approximately 50% of the pressure at the plane of its inlet nozzle. At discharge nozzle pressures below 50% of the inlet nozzle pressure, the discharge nozzle is operating in a "sonic" or "choked flow" condition in which the flow through the discharge nozzle is only affected by the inlet nozzle pressure and the physical design of the inlet nozzle.
The flow of liquids through a discharge nozzle is affected by the pressure at its discharge nozzle at all operating pressures. As a result, subsonic flow exits at all operating pressures of a right-angle body valve flowing liquids.
The evaluation of the efficiency of flow through a valve requires the evaluation of the valve's dimensions and component movements. The parameters evaluated in a right-angle body valve include the "lift," "lift ratio," "discharge coefficient" or "Kd" and "pressure ratio." These parameters may be best described with reference to FIGS. 1 and 2 illustrating a conventional right angle body valve 10.
The valve 10 is employed to convey pressurized fluid in a containment area indicated at 11 to a lower pressure outlet area indicated at 12. A circular valve seat plate or disc 13 positioned against a circular inlet nozzle seat top edge 14 prevents flow between the areas 11 and 12. The seat plate 13 is moved into and out of engagement with the nozzle edge 14 by vertical movement of a valve stem 15 connected to a conventional regulator control system indicated generally at 16. Only a portion of the regulator control system 16 is illustrated in FIGS. 1 and 2. Operation of the valve 10 is conventional with the pressure in the regulator control system 16 employed to regulate the opening and closing of the valve 10 by appropriately raising or lowering the valve plate 13. Fluid entering the open valve 10 flows through an inlet nozzle 17 and exits the valve through an outlet nozzle 18. The body of the valve and the components connected to the valve inlet and outlet form a constraining structure that alters the flow of the contained fluid between an inlet area 19 and the outlet area 12. In the case of a right-angle body valve such as the valve 10, the direction of flow is changed by approximately 90.degree..
With reference to FIG. 2, the "lift" of the valve 10 is the separating distance between the bottom face 13a of the valve plate and the nozzle seat top edge 14. The "lift ratio" for the valve 10 is the ratio of the lift to the diameter of the nozzle 17. The "discharge coefficient" or "Kd" is the ratio between the actual measured flow through the valve 10 verses the maximum theoretical flow of an ideal inlet nozzle with a bore area equal to that of the valve's inlet nozzle 17. The Kd may be measured at more than one lift ratio value. "Pressure ratio" is the ratio of the pressure at the valve's discharge point to the pressure at the valve's nozzle inlet.
It will be understood by reference to FIGS. 1 and 2 that the valve 10 and the associated pressure area 11 form a flow constraining structure that alters the direction of flow of fluid in the structure from an inlet 19 to the outlet 12.
Historically, conventional forward flow, right-angle closed body style valves operating in the subsonic flow region and at high pressure ratios have required high lifts to overcome low Kds and attain the maximum flow rates desired. For reference, typically, right angle valves flowing vapors in the sonic flow condition (pressure ratio below 0.50) will attain valve Kds in the range of 0.97 at lift ratios ranging from 25% to 45%. The same valve flowing vapors in the subsonic range will require lifts exceeding 65% to attain the same Kd value when operating at pressure ratios near the 0.50 transition point. The lift requirement increases to even higher required lifts in an environment of lower attainable Kd values as the flowing pressure ratios approach the 1.00 value.
Low subsonic Kds have been attributed to two primary factors. The first factor is an identified flow pattern that creates a flow restriction in the radial flow region between the face of the valve seat and the circumference of the valve nozzle. This restriction is created from the turning of the discharge flow that occurs when the flow stream from a nozzle strikes a generally flat surface that is perpendicular to the centerline of the nozzle and is located just beyond the discharge face of the nozzle. The second factor specifically related to right angle valves is the difficulty of obtaining a smooth aerodynamic gas flow and pressure reduction in the valve body cavity between the valve inlet nozzle and the discharge outlet.
In open discharge valve designs utilized, for example, for various low pressure relief valves for tanks, there is no surrounding valve body or conduit to constrain the exiting flow. The primary restriction on valve flow rate for this type valve is the radial nozzle flow constriction. As a result, for this style of valve, it is possible to attain essentially the maximum theoretical flow rate of the valve nozzle independent of all other factors except atmospheric back pressure by simply increasing the valve lift. Because of the open construction of these valves, any additional cost related to the solution of the problem of restricted flow is limited to that of the valve operator costs.
A high lift combined with a large valve body cavity in closed body valves can increase valve flow efficiency for many valve applications. Such a solution, however, requires a more costly operator and increased valve body dimensions with corresponding penalties in the valve body weight and cost.
The efficiency of the fluid flow through a closed body valve may be improved by shaping a downstream cavity in the valve in a way to minimize restrictions to the inlet flow. One such design is employed in the Anderson, Greenwood Series 9000 pilot operated safety relief valve. This valve includes a cavity downstream of the valve closure mechanism that has been dimensioned and contoured to minimize resistance to flow into the downstream section of the valve. As compared with a valve lacking such features, the efficiency of fluid transfer through the valve is substantially improved.